Description
This article describes what Short Guard Interval is, how it is used on FortiAPs, and when to enable this feature.
Scope
FortiAP supporting 802.11n and 802.11ac.
Solution
A Guard Interval (GI) is a period of time between symbol transmissions. It allows reflections from previous data transmissions to settle before transmitting a new symbol. The signal content inside the GI, called Inter-Symbol Interference (ISI), is rejected by receivers. 802.11a/g OFDM transceivers use 0.8μs GIs.
The HT PHY introduced two guard intervals: 400ns (0.4μs Short Guard Interval or SGI) and 800 ns (0.8μs Long Guard Interval or LGI). Support of the 0.4μs (SGI) is optional for transmitters and receivers. The VHT PHY also supports SGI and LGI of the same timing. Figure below shows the difference between these two.
Choosing a short guard interval on an environment with multipath leads to more ISI and lower throughput. A long guard interval means more idle time on the wireless medium, which is wasteful on some other environments. A good Guard Interval helps to get the most out of throughput between AP and STA. The selection of the guard interval type depends on various factors such as the specific WiFi standard being used (e.g., 802.11n, 802.11ac), the characteristics of the wireless environment, and the desired balance between data throughput and reliability. With SGI enabled in highly reflective environments, users may experience higher packet error rates; using LGI instead is recommended in these cases. It is recommended to talk with Fortinet's Professional Services to get advice on whether or not this feature should be enabled or not on a specific environment.
A 0.4μs GI adds about 9-10% to the data rate over an 0.4μs GI. This is shown in the 802.11n (HT), 802.11ac (VHT) and 802.11ax (HE) MCS table. Differing from its predecessor, 802.11ax supports three different Guard Intervals: 0.8μs, 1.6μs and 3.2μs. The sortest GI will be used when it has been enabled on 802.11ax.
By default, Long Guard Interval (0.8μs GI) is used on FortiAP Profiles. Short Guard Interval (0.4μs GI) can be enabled on the FortiAP profile (wtp-profile) through CLI or GUI.
config wireless-controller wtp-profile
edit "<profile-name>"
config radio-1
set short-guard-interval [enable|disable]
end
config radio-2
set short-guard-interval [enable|disable]
end
config radio-3
set short-guard-interval [enable|disable]
end
config radio-4
set short-guard-interval [enable|disable]
end
next
end
Example:
In a Lab environment, Short Guard Interval was enabled on the WTP profile of a FortiAP 221E. The following was observed:
